In recent years, such monocrystals as are naturally present, which have a three-dimensional shape peculiar to the crystals, have been desired in various fields because of unknown properties thereof.
Examples of a method for producing a corundum crystal include (1) the flame fusion method (Verneuil's technique) of dropping raw material powder of the corundum crystal into oxygen and hydrogen flame and simultaneously growing crystal grains; (2) the flux method of mixing raw material powder of the corundum crystal with an appropriate flux, melting the mixture in a crucible, and precipitating/growing the crystal while cooling the melted solution slowly or precipitating/growing the crystal while applying temperature gradient to the solution in the crucible, or precipitating/growing the crystal while vaporizing the flux(refer to the non-patent literatures 1 and 2); (3) the Czochralski method of melting raw material powder of the corundum crystal in a crucible, and pulling up the crystal from melt (refer to the patent documents 1 and 2); and (4) a method of forming raw material powder of the corundum crystal into a shape, and then heating the shaped powder at a high temperature in a hydrogen gas atmosphere for a long time so as to sinter the powder (refer to the patent document 3).
In the flame fusion method (1), the growth rate of the crystal is large, so that the obtained crystal cannot be made into a high quality at ease. On the other hand, the Czochralski method (3) makes it possible to produce a crystal having a high purity. Accordingly, the crystal produced can be preferably used as a laser-oscillating material or the like. However, according to these methods, a rodlike crystal is produced respectively. Thus, at the time of putting the crystal into practical use such as a laser-oscillating material, it is necessary to cut the rodlike crystal produced into a desired shape, and further the hardness of any corundum crystal is high; accordingly, a problem that costs increase arises. Furthermore, whereas the respective corundum crystal produced by these methods does not contain any impurity, the natural corundum crystals contain impurities. Accordingly, the corundum crystal thus produced can be easily distinguished and has a drawback that the crystal is very low in value as an ornament.
According to the method (4) of shaping followed by sintering, it is unavoidable to heat powder at high temperature for a long time. Thus, a large quantity of energy is needed, so as to cause a problem that costs increase.
On the other hand, it is known that according to the flux method (2), a tabular crystal can be obtained by using, as a flux, lithium oxide-lead oxide (fluoride), aluminum fluoride/sodium, lithium oxide-tungsten oxide-lead oxide (fluoride), bismuth oxide-lanthanum oxide-lead oxide (fluoride) or the like, and precipitating/growing a crystal while cooling the melted solution slowly. However, only a thin tabular crystal can be obtained. Thus, as mentioned above, there arises a problem that costs increase when the crystal is put into practical use.
Furthermore, the growth of a crystal on abase material is difficult by any methods, and particularly the direct precipitation/growth of a crystal on a base material without using a seed crystal is very difficult. Although it is possible to attach a crystal to a base material, there is a problem of easy release of the crystal.    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 7-277893    Patent document 2: JP-A No. 6-199597    Patent document 3: JP-A No. 7-187760    Nonpatent literature 1: Elwell D., Man-made gemstones, Ellis Horwood Ltd., Chichester (1979)    Nonpatent literature 2: Elwell D., Scheel H. J., Crystal growth from high-temperature solutions, Academic Press, London (1975)